This is a broad ranging proposal to continue investigations into diverse aspects of normal or disturbed thyroid hormone economy. Major topic areas to be considered are: 1) extrathyroid factors that regulate thyroid function; 2) biochemical studies of thyroregulatory mechanisms; 3) pathways of peripheral thyroid hormone metabolism; 4) cellular effects of thyroid hormones. Included in category 1 are a variety of studies concerning the regulation of TSH secretion, and the effects of physiological manipulations and pharmacological agents thereon. Category 2 includes further studies of the interaction of specific receptors for TSH in thyroid with both TSH and thyrotropic materials of trophoblastic origin. Category 3 includes additional studies of biochemical mechanisms that influence the peripheral conversion of thyroxine to either active or inactive metabolites. Category 4 comprises an extension of studies of the effects of thyroid hormone on functions of the cell membrane.